It's here in this forum. Someone called it that, the holy grail. How to start an XT 600.
I have had mine for 3 years and i still can not start it on demand every time. Like a battle of wills, i do not always win.

There are things that make it easier:
You must set your decompression cable correctly. Research it.
If you have a dirty fuel tank and/or a leaky float needle or if your engine is not in a good condition, take the bus. Get the parts and fix it.

To start:
You must consider the temperature. Cold and choke you will need. How much you will learn. Mostly full choke. And NO THROTTLE. ALWAYS.
You must learn where to 'park' the crankshaft before you kick. Very important and not easy. You can listen to the clicks the decompressor makes.
Or you can find TDC at nearly the end of the kickstart stroke. Viewed from the side the lever should be at 7 o'clock and piston at top dead centre firing position.
Now release the lever to the top and calmly push down. If it does not start, kick again from the top. The engine stops in the right place, no need to find TDC.
Starting from hot is the same. Just no choke. And NO THROTTLE.
It is said that one can start and XT with the hand, and i believe it. There is no compression, taken care of by decompressor.
But not always. Sometimes i am not perceptive enough.
Cold; after 4 kicks with full choke and not started, it will be too rich and will not start.
Hot; 4 kicks (without choke) and it can be too rich and will not start.
At this point i want to walk away. Hey, it's not my bike, just trying to steal it...
Here i learned:
When failed starting cold, open the choke. Open and hold the throttle wide, gently kick the engine over a few times to dry it out. Now try starting again. If it starts you will need a little throttle to keep it going.
When failed starting hot, also open the throttle wide and gently kick the engine over 4 or 5 times to dry it out. You may want to operate the kill switch here to avoid a backfire. Now try starting again.
These steps may be repeated until you learn just what your XT is telling you and the engine starts.
It's also a great way to save that magnesium clutch cover from fracture due to booting the engine over compression.